If there is any way in which a function containing an `unsafe` block may be used unsafely (specifically, violating memory-safety), then that function must also be marked as unsafe.
Most things don't need unsafe code. For the things that do, you must yourself uphold the invariant that all requirements of safety are being obeyed when transitioning out of an unsafe block. If you don't do this, bad things can happen. Other languages don't have this because they either don't offer Rust's safety guarantees in the first place, or the only way to circumvent them is to write code in C.
I may be missing some context, but this is certainly not true in Rust. In order to understand whether an individual piece of code marked `unsafe` is actually correct, you need to examine the context in which it is run and in general you could have to examine a large section of "safe" code in order to figure out whether the "unsafe" block is correct. Usually you will have to examine the entire module.